1. Field of the Invention
This invention relates to a brushless DC motor, and more particularly is directed to an improved drive circuit for a brushless DC motor.
2. Description of the Prior Art
In the prior art, DC motor control circuits have been proposed which permit substantially constant torque to be developed by the motor regardless of the rotational angle of the motor shaft. For example, U.S. Pat. Nos. 3,383,574 and 3,517,289 teach the use of Hall-effect elements to control the current fed to the motor armature winding in sinusoidal relationship to the angular position of the rotor so as to obtain such substantially constant torque.
However, if, as in the above examples of the prior art, the magnetic field from the rotor is detected by two Hall-effect elements to directly produce sinusoidal currents flowing through the stator windings, a DC offset voltage is generated in the detected output from the Hall-effect elements so that a DC component is contained in the sinusoidal currents flowing through the stator windings. For this reason, the positive half-cycle interval of the sine wave is different in length from the negative half-cycle interval thereof to cause a torque ripple. The foregoing results from the fact that, even though several Hall-effect elements are made of the same material, sensitivities of the several elements are not coincident with each other but are dispersed. Accordingly, the output voltages thereof are also scattered to cause the torque ripple.
Further, it is difficult to magnetize the rotor magnet so as to make its magnetic field exactly sinusoidal, and, in the normal course of events, there will be uneven portions of the magnetization. This also causes the torque ripple.